Infra-red radiant energy devices



EMMINE 455-613 AU 233 EX 'FIPBlOb UR 3,111,587

W MW i I n iv E (MDWMQ/M Nov. 19, 1963 Y. A. ROCARD 3,111,587

INFRA-RED RADIANT ENERGY DEVICES 5 Original Filed Sept. 30, 1954 3Sheets-Sheet 1 cLXW Yvas A. Roam:

5 BY n ATTORNEY! Nov. 19, 1963 Y. A. ROCARD 3,111,587

INFRA-RED RADIANT ENERGY DEVICES Original Filed Sept. 30, 1954 3Sheets-Sheet 2 INVENTOR Yves A. R ocnno Y iza f/QW Nov. 19, 1963 Y. A.ROCARD INFRA-RED RADIANT ENERGY DEVICES Original Filed Sept. 30} 1954 3Sheets-Sheet 3 vR W WA- A i,- vm.

AMPLIFIER.

United States Patent 8 Claims. (Cl. 250199) This invention relates todevices for the generation, modulation, transmission and reception ofinfra-red energy.

This application is a division of my co-pending application Serial No.459,307, filed September 30, 1954, now abandoned.

Infra-red radiant energy has long been recognized as having manyadvantages over radiation in the visible and other regions of thespectrum for use in optical signalling systems and radiation barriersystems for scanning an area to detect the presence of objects therein.Among the advantages of infra-red are the better transparency of theatmosphere to radiation of infra-red wavelengths than to visibleradiation, which permits use of infra-red type systems even where fog,clouds or smoke prevent use of visible light systems. Another importantadvantage is that infra-red is invisible to the naked eye, hence asignal transmitted by infra-red is less easily intercepted by personsother than the intended recipient, and the presence of an infra-red typeradiation barrier system less easily detected by intruders.

More extensive use of signalling and radiation barrier systems of thistype has failed to materialize primarily because the systems heretoforeknown have employed infra-red sources having outputs which could noteasily be modulated according to the signal to be transmitted or easilycollected into narrow beams of predetermined geometric form. Probablythe most commonly used of the known infra-red sources is theincandescent filament or glower, which consists of a metallic or otherconductor heated to incandescence by passage of an electrical currenttherethrough. Due to the large heat capacity of such sources theirradiation output lags far behind the heater current and therefore cannotbe modulated by varying heater current amplitude except at very lowfrequencies. The glow element must have a relatively large surface areafor emission of adequate quantities of infrared, hence it does notconstitute a pin-point or quasi pinpoint source and must be providedwith a complex optical system if the radiation emitted thereby is to becollected into a narrow beam of parallel rays as required for mostsignalling and radiation barrier systems. Moreover, such a source emitsvisible as well as infra-red radiation and for many applications must beprovided with a filter for absorbing this visible radiation. Such filteradds to the cost of the system and, by absorbing some infra-redradiation as well as the visible, to its inefiiciency.

In accordance with the present invention it is possible to obviate theabove described disadvantages of known infra-red signalling andradiation barrier systems and to improve the efliciency and usefulnessof such systems, by use of infra-red radiant energy sources wherein amodulated electric current is passed through a point contact togermanium or other semiconductor material to cause emission therein ofinfra-red radiation modulated correspondingly to the current.

It has been previously reported that if an electric current is broughtthrough an electrode having point contact with a body of semiconductormaterial such as germanium, the electrons in the material are attractedtowards the point, thereby leaving holes in the semiconductor material.The recombination of electrons from the negative electrode with theseholes causes the emission of infrared radiation at a wave length ofabout 2 to 3 microns. This phenomenon is evident in most semiconductorsunder the same conditions; i.e., the injection of positive holes in asemiconductor of the N-type or the injection of electrons in asemiconductor of the P-type will cause this eifect.

My invention is directed towards devices utilizing this infra-redemission phenomenon for the generation, modulation, transmission andreception of infra-red radiant energy and for the establishment ofradiation barriers for scanning an area to sense the presence of objectstherein.

It is accordingly a primary object of this invention to provide new andimproved infra-red signalling and radiation barrier systems.

It is also a primary object of this invention to provide new andimproved semiconductor infra-red sources capable of emitting andmodulating infra-red radiant energy without accompanying emission ofvisible light.

A further major object of this invention is the provision of novelinfra-red emitting devices capable of producing infra-red radiation andof directing the radiation produced into radiant energy beams ofpredetermined geometric form.

Another important object of this invention is to provide novelsemiconductor infrared emitting devices with several emission points ona single piece of semiconductor material, which emission points may beeither similarly or differently modulated.

It is also an object of this invention to provide new and improvedsemiconductor infra-red energy emitting units containing materials, inaddition to the semiconductor material, for providing a broaderinfra-red emission spectrum and optimum operating impedance, and also toprovide units that can be easily manufactured.

Another object of this invention is to provide novel semiconductorinfra-red emitting devices in which the semiconductor material isgeometrically shaped in a manner to provide optimum output of parallelbeams without the use of accessory optical equipment.

A further object of this invention is to provide semiconductor infra-redemitting devices in combination with focusing and receiving units toform new and improved signalling and signal alignment systems.

It is another object of this invention to provide new and improvedradiation barrier systems utilizing semiconductor infra-red emittingdevices- It is also an object of this invention to provide novel meansfor pulsing radiation reflected from a photoreflector by means ofmechanical shutters.

These and other objects, features and advantages of the presentinvention will become more fully apparent by reference to the appendedclaims and the following detailed description when read in conjunctionwith the accompanying drawings, wherein:

FIGURE 1 is a diagrammatic representation of an arrangement forproducing infra-red energy by passing an electrical cur-rent through abody of semiconductor material;

FIGURE 2 is a diagrammatic representation of a device for modulating theinfra-red energy emitted from a single point contact to a body ofsemiconductor material;

FIGURE 3 is a diagrammatic representation of a device for modulating theenergy emitted from more than one point contact on a piece ofsemiconductor material;

FIGURE 4 is a diagrammatic representation of a device employing twounattached semiconductor units for producing an extremely fine beam ofinfrared energy which extends between two areas of modulated infra-redenergy and which is itself either non-modulated or modulated in a mannerdifferent from the energy in said areas;

FIGURE 5 is a diagram showing the pattern of radiation emitted by thedevice represented by FIGURE 4;

FIGURE 6 is a diagrammatic representation of a de- 3 vice employing asingle semiconductor unit having two point contact electrodes forproducing an extremely fine beam of infra-red energy which may or maynot be modulated and which extends between two areas of modulatedinfra-red energy;

FIGURE 7 is a diagram showing the pattern of radiation emitted by thedevice represented by FIGURE 6;

FIGURE 8 is a diagrammatic representationof a semiconductor infra-redemitting device employing four point contacts on the surface of a singlecrystal of semiconductor material;

FIGURE 9 is a diagrammatic representation of a semiconductor device fortransmitting modulated infra-red radiant energy and a device forreceiving said energy by means of a cooled infra-red sensitivephotocell;

FIGURE 10 is a diagrammatic representation of a semiconductor infra-redemitting device employing a piece of indium placed under the pointcontact of the semiconductor material;

FIGURE 11 is a diagrammatic representation of a piece of semiconductormaterial employing three indium pellets embedded in the surface of saidmaterial under a like number of point contact electrodes;

FIGURE 12 is a diagrammatic representation of a semiconductor infra-redemitting device shaped in the form of a Cartesian oval section;

FIGURE 13 is a diagrammatic representation of a semiconductor infra-redemitting device shaped in the form of a sphere section;

FIGURE 14 is a diagrammatic representation of a semiconductor infra-redemitting device of conical shape with a radiation exit surface in theform of a sphere section of geometric proportions similar to theCartesian oval section;

FIGURE 14A is a sectional view of a semiconductor infra-red sourcehaving a modified point and base electrode arrangement;

FIGURE 15 is a diagrammatic representation of an infra-red barrierdevice without optical components for the scanning of an object or areawithin a short distance;

FIGURES 16 and 17 are diagrammatic representations of photoradiatorssuitable for use in the radiation barrier and signal alignment systemsof this invention;

FIGURE 18 is a diagrammatic representation of a radiation barrier deviceemploying a semiconductor infra-red source along with photoradiators andan infrared sensitive photocell;

FIGURE 19 is a diagrammatic representation of a barrier device employinga series of infra-red sensitive photocells concentrically placed arounda semiconductor infra-red source;

FIGURE 20 is a diagrammatic representation of a barrier device employinga modulated semiconductor infra-red source, two photoradiators and aninfra-red sensitive photocell placed at the focal point of a mirror;

FIGURE 21 is a diagrammatic representation of a device for pulsing theradiation reflected from the photorradiator by means of mechanicalshutters;

FIGURE 22 is a diagrammatic representation of a device for transmittinginfra-red energy emitted from a semiconductor source, a means foraccurately aiming this device by use of an incandescent infra-red sourceand an electronic telescope, and means for receiving the modulatedsignal from the semiconductor source by use of an infra-red sensitivephotocell placed at the focal point of a mirror;

FIGURE 23 is a diagrammatic representation of a device for modulatingthe infra-red energy emitted from a semiconductor source by the use of acarbon microphone placed in series with a battery and the point contactof the germanium crystal;

FIGURE 24 is a diagrammatic representation of a device for projecting afine beam of modulated infra-red radiant energy from a semiconductorsource and incorporating a device for aligning said projector by meansof an incandescent infra-red source and an infra-red filter; and

FIGURE 25 is a diagrammatic representation of a receiving deviceemploying an infra-red sensitive photocell at the focus of a mirrormounted on a support, said support also holding a photoradiator for thepurpose of reflecting some of the infra-red energy back to thetransmitting station for purposes of alignment of the transmittingdevice with the receiving device.

With continued reference to the drawings, wherein like referencenumerals are used throughout to indicate like parts, FIGURE 1 shows aninfra-red emitting device comprising a crystal or pellet 30 of asemiconductor material such as N-type germanium, a positive electrode 31having point contact with the germanium crystal 30 and a negative orbase electrode 32 having large area contact therewith, the positive andnegative electrodes being connected with a DC. current source 33. Thepositive point contact electrode 31 causes a migration of electrons inthe germanium towards the positive electrode with a consequent formationof holes in the germanium 30. Electrons from the negative or baseelectrode 32 then enter the germanium and combine with the holes. Thiscombination of electrons with the holes" occurs with an emission ofinfra-red energy at approximately 1.8 to 3 microns.

FIGURE 2 shows a semiconductor infra-red source the infra-red radiantenergy emission of which is modulated in accordance with this invention.Speaking into microphone 35 causes a fluctuation of current from the D.C. source 40 to pass through the primary coil of transformer 42. Thesecondary coil of the transformer will then modulate the output of DC.polarizing voltage source 43, and the current flowing from the negativeor base electrode 36 into semiconductor crystal 37 will modulate theelectron flow to point contact electrode 38, thus causing acorrespondingly modulated infra-red emission from the semiconductorcrystal. It is important that the polarity of the device remainunchanged so that the direction of electron flow within thesemiconductor does not change. Uni-directional flow of current mayeasily be maintained by making the DC. polarizing voltage source 43sufficiently high that it will always be greater than any voltage ofopposite sign generated in the transformer 42 or other current modulatorin the circuit. The optimum operating current for the semiconductorunits is about 500 to 800 milliampcres at 0,5 to 1.0 volt.

As noted above, the infra-red emission phenomenon is a general oneevident in most semiconductors under the same conditions. Whilegermanium of either the N- or P-type is the preferred semiconductormaterial for use in the devices of this invention, other semiconductorssuch, for example, as silicon carbide (the emission spectrum of whichextends into the visible), cadmium sulfide and other binary metalcompounds having characteristic infrared emission may also be used. Ifthe semiconductor to be used is of the N-type the point contactelectrode should be connected to the positive side of the DC. polarizingvoltage source and if of the P-type to the negative side thereof.

The infra-red emission of semiconductor units such as shown in FIGURE 2may be modulated up to very high frequencies (of the order of megacyclesper second). The radiation emitted by germanium is only in the infraredregion of the spectrum and includes no radiation of wave lengths withinthe visible region. This type of infra-red source may therefore be usedwithout the visible light filter necessary to the usual infra-redprojection devices. Because the semiconductor material is transparent tothe infra-red radiation formed by the combination of electrons and holestherein, the radiation can escape from the semiconductor for projectionpurposes.

For some applications it may be advantageous to shape the semiconductorcrystal as shown in FIGURE 2 or to leave it in the shape in whichoriginally found or produced, so that the radiation emitted within thecrystal will not be refracted or reflected by its passage through theexit surfaces thereof into a beam of particular geometric pattern.Because substantially all of the infra-red radiation is emitted in theimmediate vicinity of the point contact electrode, however, it isreadily possible to focus the emitted radiation into a projected beam ofany desired geometric form. This may be done by forming the crystal in amanner such that its exit surface has an optimum geometric shape, asdetremined by the general principles of optics, for the projection ofnarrow beams of infra-red radiation.

If the infra-red emitter is to be used in combination with a mirror orlens for focusing the emitted radiation, the exit surface of thesemiconductor body through which the radiation escapes is preferably inthe form of a segment of a sphere the center of which is at or near thepoint contact electrode, so that each of the infra-red rays emitted atthe point contact will be normal to the exit surface at the point atwhich it passes through that surface. There will then be no refractionor bending of the rays due to the high index of refraction of thesemiconductor material, and the emitter will constitute an apparentpin-point source which, when placed at the focal point of a mirror orlens, will provide a very narrow beam of parallel rays withoutappreciable aperture. An emitter and mirror system of this type is shownin FIG- URE 9, for example. While mirrors are used in this illustration,it is not essential that mirrors or lenses be used and if the exitsurface of the emitter is shaped in the manner described hereinafterwith particular reference to FIGURES 12-14, no mirror or lens isnecessary to obtain such a narrow beam.

In FIGURE 3 a germanium crystal 44 is employed as an infra-red emitterutilizing several point contacts 46 for the positive electrodes. Severalpoints are used instead of a single point to increase the infra-redradiation output of the germanium crystal. The point contacts are shownwidely spaced from each other in FIGURE 3 only for purposes of clarityof illustration; in practice the several points are placed closelytogether so that the geometric form of the germanium crystal, and itsassociated optical system if any, can focus the infra-red radiationemitted from all the points. An oscillator 48 emits a signal whichpasses through transformer 42 andmodulates the current input from theDC. source 43. Electrons within the germanium migrate towards thepositive point contact electrodes 46 and new electrons enter thegermanium crystal through the negative electrode 50. The combination ofthe electrons entering the crystal through electrode 50 with the holes"produced by the migration of electrons already in the crystal towardpoint contacts 46 produces the infra-red energy which is emitted asshown by dotted lines 52.

In FIGURE 4, two germanium crystals 54 and 56 are placed at the focalpoint of a mirror 58. A signal of 400 cycles per second is emitted byoscillator 60, thus modulating the current entering the germaniumcrystal 54. Another oscillator 62 modulates the current entering crystal56. This second oscillator emits a signal of 1,000 cycles per second ofthe same intensity as the signal emitted by oscillator 60. The modulatedinfra-red energy output of the crystals 54 and 56 impinges on mirror 58.Two modulated infra-red beams are reflected, one at 400 and the other at1,000 cycles per second and the intersection of these two beamscomprises a narrow beam 64 which is made up of 400 and 1,000 cyclesignals of equal intensities.

FIGURE 5 shows diagrammatically the areas covered by the infra-redenergy reflected by mirror 58 in FIG- URE 4. Area 66 represents the 400cycle beam. Area 68 represents the 1,000 cycle beam. The beam in whichthe 400 and 1,000 cycle signals are of equal intensity is indicated byline 64. Such a beam may be used as an aid to navigation of aircraft ornaval vessels, the beam being directed to mark the path along which theplane or ship is to be guided. An infra-red detector carried in theplane or ship will detect 400 and 1,000 signals of equal intensities solong as the vessel remains on course, but either the 400 or the 1,000cycle signal will predominate over the other should the vessel wanderoff course. This system is particularly effective where the obstructionto normal vision is due to fog, clouds or the like, since these arerelatively transparent to infra-red radiation.

The device of FIGURES 4 and 5 may also be used as an easily alignedtransmitter for an infra-red signalling system, such transmitter beingadjusted until the receiver at which its beam is directed detects 400and 1,000 cycle signals of equal intensity. The transmitter and receiverare then properly aligned for best signal transmission and reception.

In FIGURE 6 a single germanium crystal 70 having two point contactelectrodes at the focal point of mirror 58 provides substantially thesame result as obtained from the two crystals of germanium used in theapparatus represented by FIGURE 4. The oscillator 72 emits a signal of400 cycles per second through point contact electrode 74 into thegermanium crystal 70 while oscillator 76 emits a signal of 1,000 cyclesper second through the point 78 into the germanium 70. The modulatedinfrared energy emitted at the points 74 and 78 impinges on mirror 58and is reflected in the pattern shown in FIG- URE 7.

The multi-point single crystal system of FIGURE 6 is, in general,interchangeable with the multiple crystal system of FIGURE 4 and can beused in the same manner and for the same purposes as that system. Acomparison of the signal patterns shown in FIGURES 5 and 7 shows thedifferent systems to have similar signal outputs.

The device represented by FIGURE 6 can also be operated in anothermanner. The output of oscillators 72 and 76 may be modulated at the samefrequency, for example, 400 cycles per second. The output of oscillator72, however, may be so modulated that the signal from point 74 isemitted as short pulses, or dots. The output of oscillator 76 may thenbe modulated in such a manner that the signal from point 78 is emittedas long pulses, or dashes, complementary to the short pulses or dotsemitted from the point 74.

In FIGURE 7 area 80 represents the infra-red energy emit-ted fromcrystal 70 at point 74 which has been reflected by the mirror 58. Area82 represents the infrared energy emitted by crystal 70 at point 74which has been reflected by the mirror 58. A detector placed in area 80would receive dots at 400 cycles per second, and one placed in area 82would receive dashes at 400 cycles per second. A detector placed in thearea represented by line 84 would receive a continuous signal made up ofthe dashes emitted from the point 78 and the complementary dots emittedfrom point 74.

The device of FIGURE 4 may also be operated in this manner by settingoscillators 60 and 62 (FIGURE 4) at the same frequency with oneproducing dots and the other producing dashes complementary to the dots.

The systems of FIGURES 4 and 6 when arranged for dot-dash operation maybe used in much the same manner as described above with reference to 400cyclecycle operation, the only difference being that with dot-dashoperation a. detector kept on the line of intersection of the tworadiant energy beams receives a continuous signal of a single frequency,rather than two signals of different frequencies. When the detector isdisplaced from such line it receives a discontinuous signal made up ofeither dots or dashes, depending on which side of the line it isdisplaced to.

In FIGURE 8 a germanium crystal 86 is placed at the focal point of theparabolic mirror 88. The crystal has four infra-red emitter points 90,91, 92 and 93. By employing the procedures described hereinabove, theoutput of each point can be modulated at a different frequency or at thesame frequency but with different pulse durations. For example, theoutput of points 90 and 92 could be 400 cycles per second with one pointemitting dots and the other dashes complementary to the dots. The outputof points 91 and 93 could be modulated at 2,000 cycles per second withone point emitting dots and the other complementary dashes. At thereceiving station a high frequency signal (2,000 c.p.s.) and a lowfrequency signal (400 c.p.s.) would be detected around an axis which isa straight line in space at approximately the axis of the parabolicmirror 88. The signal reflected at the axis of the mirror would bereceived as a continuous signal by a photosensitive cell. This systemgives an indication of alignment in two planes, rather than in a singleplane as in the devices of FIGURES 4 and 6. Thus, the beams emitted bypoints 90 and 92 in FIGURE 8 may be used for horizontal alignment, andthose emitted by points 91 and 93 for vertical alignment. Such anarrangement is of particular utility in guiding aircraft as for examplein defining the desired glide path for aircraft when landing under poorvisibility conditions.

It has been determined that parabolic mirrors are the most efficient inthe above discussed devices. Satisfactory materials for construction ofthese mirrors are copper, zinc, silver or aluminum, all of which arecapable of efficiently reflecting infra-red radiation. Glass mirrorswith a first surface coating of aluminum or gold are also satisfactory.Any lenses used in the infra-red transmitting or receiving devicesdescribed herein should preferably be constructed of materials havingmaximum transmission properties in the infra-red region, thalliumbromide or iodide or mixtures of the two, or silver chloride, beingsatisfactory.

In FIGURE 9, the infra-red output of germanium source 95 is modulated bymicrophone 35. The emitted infra-red radiation strikes mirror 96 and isreflected onto mirror 97, by which it is directed to the photoconductivesurface 98 of photocell 100. The resistance of the photoconductivesurface 98 changes with variations in the impinging infra-red radiationand the modulated output of the cell is amplified by amplifier 102 anddelivered to a speaker 104.

The germanium source 95 has an infra-red emission spectrum with a peakat 2.5 microns and emits no visible light. Lead sulfide cellsmanufactured as described in my United States Letters Patent No.2,884,345, issued April 28, 1959, are sensitive up to 3.5 microns atroom temperature and up to 4 microns at the temperature of liquid air(180 C.). The lead telluride cells described in United States LettersPatent No. 2,858,398, to Rocard et al., issued October 28, 1958, aresensitive to 6 microns when cooled to the temperature of liquid air(-180" Either cell can be used in the device represented by FIG- URE 9or in any of the devices described herein which employ an infra-redresponsive photoconductive cell.

FIGURE 9 shows the photocell as provided with means 105 for cooling itto the temperature of liquid air, the cooling means 105 comprising ahollow-walled flask inserted into one end of the photocell and filledwith liquid oxygen or other coolant.

It is important to note that the efficiency of the device represented byFIGURE 9 is high because no infra-red filter or lenses are necessary forthe transmission or reception of the infra-red radiation. In devicespreviously reported, an incandescent infra-red source such as a tungstenfilament lamp was employed and this necessitated the use of filters toabsorb the visible light. An optically shaped germanium source, on theother hand, will project a beam without the filter or lenses necessaryin such other types of infra-red projection apparatus.

It has been previously reported that the deposition of indium in pelletor layer form on an N-type germanium crystal directly under the pointelectrode contact increases the hole density in the area adjoining saidcontact. I have found that this increase in hole density results in acorresponding increase in emission of infrared radiation in the vicinityof the point contact. Although indium is the preferred metal for use inthe devices to be described hereinbelow, other metals can also beemployed, such as zinc, cadmium, silver and copper.

In FIGURE 10, an N-type germanium crystal 106 is employed as aninfra-red emitter. An indium inclusion 108 is placed in or on thesurface of the crystal directly under the point contact electrode bymechanical means or by electrodeposition.

The surface of the crystal may conveniently be prepared and the indiumdeposited thereon in the manner outlined in the article entitledElectrochemical Techniques for the Fabrication of Surface-BarrierTransistors" appearing in the December 1953 issue of I.R.E. Proceedings,at pages l702-1708.

A modulated signal from oscillator 110 causes the included metal at 108to become more positive, attracting electrons from the germanium. Thisincreases the hole population throughout the germanium, but particularlyin area 110. In effect, then, area 110 becomes a P-type germaniumsection. The inclusion of the pellet or layer of indium or other metalwith similar properties under the point contact is advantageous becausethe infra-red output of the crystal is increased, the units are easierto manufacture, and the resulting impedance values of the device aremore satisfactory for instrumentation purposes than those of puregermanium.

It is also possible to produce multiple electrode structures on a singlegermanium crystal blank as shown in FIGURE ll. Indium underlays 112, 113and 114 are set into the hemisphere of the N-type germanium crystal 116directly under the positive point contact electrodes and the negativeelectrode 118 is positioned on the side of the hemisphere. The indiuminclusions should be located sufficiently close to the center of theshaped crystal so that the infra-red radiation is emitted approximatelynormal to the crystal surface. Infra-red sources of this type includingindium or similar metal underlays between the point contacts and thesemiconductor material (in this case, germanium) may be employed in thevarious devices described herein.

We may assume that the emission area in the germanuim source is almost apin point. With such an assumption we may then determine the bestprocedure for projection of the infra-red energy in a narrow or broadbeam. One method is to use a hemisphere shaped germanium sourcepositioned at the focal point of a mirror which gathers and reflects theinfra-red energy within a wide angle. Such a source has been describedhereinabove and is illustrated in FIGURE 9. It is also possible to focusthe infra-red radiation emitted from the germanium source by propershaping of the germanium crystal. For example, if emission takes placeat 120 in FIGURE 12, the portion of the germanium which allows the exitof a parallel beam from point 120 is a section 121 of the Cartesian oval122. With the Cartesian oval section it is not necessary to use a mirroror lens to protect a narrow parallel beam of infrared energy from thecrystal.

The index of refraction of germanium in the infra-red region isapproximately four and calculations indicate that with such an index ofrefraction, the Cartesian oval described above is almost a sphere. Thus,a practical solution in the production of the infra-red sources is touse a cross-section 124 (FIGURE 13) of a germanium sphere cut slightlybeyond its center at 125. The point electrode is placed at 126. Theplane at which the sphere of germanium should be cut to obtain thedesired focus of emitted infra-red energy may be determined by infra-redmeasurements on the emission from the sphere as it is being shaped, andalso may be determined mathematically.

The proper cutting plane of a spherical crystal is given by the formula:

where S is the distance between the cutting plane and the meridian planeparallel thereto; r is the raidus of the spherical surface; and n is theindex of refraction of the material of which the crystal is made. Forthe particular semiconductor germanium, the cutting plane is spaced fromthe parallel meridian plane a distance equal to about one-third theradius of the crystal, since the index of refraction of germanium is inthe neighborhood of four for infra-red wave lengths.

It is also possible to formv in effect an achromatic doublet at theoptically shaped emitting surface of the source unit bycapping thesurface with a shaped silicon lens, whereby the radiant energy emissionfrom the surface is more finely focused.

FIGURE 14 shows the use of a conical cross-section 128 of either asphere or a Cartesian oval. The point contact 130 with an indium orsimilar metal underlay is at the apex of the cone, permitting theemission of parallel beams of infra-red energy as represented by dottedlines 131. This shape allows use of a minimum amount of material andresults in the impedance of the unit being higher than in the germaniumhemisphere. Such a conical section projects parallel beams of infra-redradiation quite satisfactorily, even though there may be some loss oflateral radiation at the point contact.

The semiconductor infra-red source illustrated in FIG- URE 14A issimilar in form to that of FIGURE 14, but differs therefrom in its pointcontact and base electrode arrangement. In FIGURE 14A, the conicallyshaped germanium crystal 260 as shown has an indium or similar metalinlay 262 at the apical end thereof and a spherical or otherwiseoptically curved exit surface 264'opposite the inlay 262. The baseelectrode in this embodiment consists of a metal plate 266 having acentral aperture 267 therethrough adapted to fit snugly about thecrystal 260, to thus provide good electrical contact between the crystaland plate. The point electrode consists of a pointed member 268 whichresiliently presses against indium inlay 262 or directly against crystal260, and conveniently may be mounted to one end of a'leaf spring 270 theother end of which is fastened to but electrically insulated from thebase contact plate 266 as by an insulating washer 272 interposedtherebetween. The point and base contact assembly 266-272 is removablysecured to germanium crystal 260 by bolts 274 passing through aninfra-red transparent plate 276 provided with a central recess 278therein shaped complementarily to the optically curved exit surface 264of crystal 260. If desired, the infra-red transparent plate 276 may beof silicon and have its center portion shaped to provide an achromaticdoublet as described above.

The electrode arrangement of FIGURE 14A is of particular advantage inthat it permits limited adjustment of the base and point contactelectrodes, as by slightly varying the diameter of the central aperture267 in base electrode plate 266 and/or shifting point contact spring 270with respect to the base electrode plate, and thus enables positioningthe electrodes to obtain maximum radiant energy emission. A further andequally important advantage in the device of FIGURE 14 resides in itsfreedom from soldered electrodes; soldering operations frequently causeundesirable change in characteristics of the germanium or other crystaland for this reason are to be avoided.

The ease of modulation of the germanium source coupled with its largeinfra-red radiation output permits the use of such sources in devicesfor transmitting and receiving infra-red radiation over long distances.Because of the very low time constants of the sources and receivingcells, the devices perform even more satisfactorily than 10 normaltelephonic communication equipment. The narrowness of the infra-redenergy beam emitted from the source makes it preferable to transmit thesignals between two fixed points or to utilize special supplementaryequipment for aiming the transmitter at the receiver, or vice versa.

In addition to providing a highly satisfactory communication system, thedevice represented by FIGURE 9 may also be employed as a radiationbarrier by utilizing a fixed frequency signal at the source 30 insteadof the signal modulated by microphone 38.

FIGURE 15 illustrates a radiation barrier designed for short distances(about three meters) employed as a protective device without the needfor optical accessories. A source 132 behind one of the walls 134projects a parallel beam of infra-red radiation represented by lines 135onto a photoconductive layer 136 of cell 138. An object 140 passing inthe area between walls 134 will cause a change in conductivity of celllayer 136 and the signal thus produced is amplified by amplifier 141 toactuate a meter or alarm mechanism 142. This unit may thus be used toprotect a corridor, door, safe, etc., in a relatively simple andundetectable manner.

The term photoradiator as used herein refers to a device which reflectsa projected beam of radiation directly back to its source with a minimumloss of energy. FIGURE 16 shows a reflecting device of this type, madeup of three plane mirrors 144-146 which are preferably, but notnecessarily, first surface mirrors. The three mirrors are disposedperpendicularly to each other and are interconnected into a unitaryassembly as shown. FIG- URE 17 shows a photoradiator similar in functionto that of FIGURE 16 but employing a prism 148 rather than mirrors, theprism being a reflecting prism of tetrahedonal form and having a frontface in the form of an equilateral triangle arranged normal to theradiation which the prism is to reflect. The prism should be fabricatedof a material which is transparent to infra-red and which has an indexof refraction such that the desired specular reflection occurs at thereflecting faces of the prism. These reflecting faces, namely, the threefaces of the prism which extend rearwardly of the face arranged normalto the impinging radiation, are congruent isosceles triangles the basedimension of which is selected to provide a prism of desired frontalarea and the altitude of which is such that each reflecting face lies ina plane disposed at an angle of 45 to the plane of the front face of theprism. It can easily be shown that to meet this requirement it isnecessary that the altitude of each triangular reflecting face beapproximately 0.408 times the base dimension thereof and that the baseangles be approximately 39" 13'. Specular reflection will then occur atthe reflecting face first impinged upon by any one ray, to turn that raythrough an angle of 90 and onto a second reflecting surface where it isagain turned through 90 and thus aimed directly back at its point oforigin. Photoradiators of this type are described and claimed in myapplication Serial No. 399,674, filed December 22, 1953, now abandoned.

A radiation barrier employing one of these photoradiators is shown inFIGURE 18 and includes in the active station the transmitter 150 andreceiver 152. The passive station consists of the photoradiators" 153and 154 which may be of either the mirror type shown in FIGURE 16 or theprism type shown in FIGURE 17. Only the active station is initiallyadjusted and no further maintenance is necessary. The projected andreflected beams of infra-red energy constitute the radiation barmier.Because of the dimensions and characteristics peculiar to the germaniuminfra-red source, the barrier area is different from that reported in myabove mentioned application Serial No. 399,674.

The signal emitted from oscillator 156 causes a modulated current frombattery 157 to pass through germanium source 150 located in shield 158.The modulated infra-red beam thus produced is projected on tophotoradiators 153 and 154 which reflect the energy back tophotoconductive infra-red sensitive cell 152. So long as the signalimpinging on cell 152 is constant, the circuit of the alarm device 160remains open. If the radiation between the active and passive stationsis interrupted, the change in signal received by cell 152 is amplifiedby amplifier 161 and actuates the meter or alarm mechanism 160.

Because the source 150 is very small, the photoradiators 153 and 154will reflect beams equal to a circle of radius R where R is equal to thelargest useful dimension of the photoradiator. Cost considerations makeit usually impractical to construct very large photoradiators,especially for infra-red radiation and an R of 3 cm. has

een found quite satisfactory for most purposes.

For maximum utilization of the energy reflected from thephotoradiators," an arrangement such as shown in FIGURE 19 may beemployed. Photoconductive infrared sensitive cells 163, 164, 165, etc.,may be placed concentric to infra-red source 166, within radius R toutilize the area 1rR with maximum efficiency. Another satisfactoryprocedure for utilizing all the energy in this area vrR is illustratedby FIGURE 20.

In FIGURE 20 the photoconductive infra-red sensitive cell 167 is placedat the focal point of mirror 169, whose optimum diameter is 2R where Ris the maximum useful dimensionof the photoradiators" 171 and 172. Theinfra-red energy from the germanium source 174 in shield 176 placeddirectly behind the photocell is modulated by oscillator 178 and isprojected on to photoradiators 171 and 172. The infra-red energyrepresented 'by lines 180 is then reflected from the photoradiators backto mirror 169 and onto the photoconductive surface of cell 167 locatedat the focal point of mirror 169. If an object interrupts the barrierthus formed by the projected and reflected infra-red radiation, a changein photoconductivity results in cell 167 and the signal thus produced isamplified by amplifier 182 to actuate meter or mechanism 183.

FIGURE 21 shows a system for pulsing the radiation reflected from thephotoradiators. Radiation projected by a source such as an incandescentlamp, filtered or unfiltered, or a germanium crystal, impinges onphotoradiators 184 and 185 located in housing 186 and is reflected backto an area determined by the positioning of the photoradiators. Theprojected and reflected radiation can then be pulsed with mechanicalshutters 188 to produce a signal or scott noise for reception by areceiving station located either at the source of the radiation orelsewhere. This arrangement permits two-way communication betweenstations only one of which is provided with a radiant energy projector.

While there are many advantages to the narrow, pin point beams ofradiation provided by the germanium infra-red emitters describedhereinabove, the very narrowness f the beams renders it more difficultto properly align them with the receiving stations to which theirsignals are directed. FIGURE 22 shows a device that will greatlyfacilitate accurate aiming of the infra-red eam emitted from a germaniuminfra-red source. There are essentially four units in this device. Thefirst is an electronic telescope" 190 of conventional type, composed ofobjective lens 192, image converter 194, fluorescent screen 196 andocular 198. The second unit is a germanium infra-red source 200containing the germanium crystal 202 properly housed as disclosed forexample in FIGURE 18 and firmly mounted on telescope 190. The third unitis a high power incandescent infrared beacon 204 composed of anincandescent lamp 205, mounted at the focal point of concave mirror 207,and an infra-red filter 208. The fourth unit is an infra-red receiver210 composed of an infra-red sensitive photoconductive cell 212, whichmay be of the lead sulfide type, mounted at the focal point of mirror214. The signal 12 received by this unit is fed into an amplifier 216and then to'speaker or headphones 217.

A device such as that represented by FIGURE 22 is placed at both thereceiving and transmitting stations located at widely separated pointsand the following procedure used to align the devices for messagetransmission by modulated infra-red radiation. Each station lights itsinfra-red beacon 204 and the electronic telescope" is used to detect thebeacon from the opposite station. Cross hairs located at the ocular ofthe electronic telescope allow proper positioning of the instrument inthe center of the field of radiation projected by the beacon 204 fromthe opposite station. When the beacon of one station is properlycentered in the cross hairs of the electronic telescope of the otherstation, the germanium source 200 of the transmitting station is in linewith the receiving unit of the receiving station. Speaking into themicrophone 218 of the transmitting station causes a modulation of theinfra-red energy output of the germanium source 200 and this modulatedinfra-red signal is received by the receiver 210 of the oppositestation. The photoconductivity of cell 212 changes with the variationscaused by impinging infra-red radiation and the resulting signal isamplified by amplifier 216 and is then passed into speaker or earphones217. Optical devices such as additional mirrors or lenses may be addedto the germanium source 200 or to the receiver 210 if desired.

The device represented by FIGURE 22 may also be used to set up aradiation barrier between itself and photoradiators placed at a distantlocation. The technique described above may then be used to align thedevices so that the radiation emitted by the germanium source 200impinges on the photoradiators and is reflected back to the receiverunit 210.

As noted above, the germanium infra-red source requires a current ofabout 500 to 800 milliamperes per point for satisfactory operation. Whenan indium layer or insert is placed between the point and the germanium,the current requirements are substantially higher and the germaniumsource thus requires a DC. power supply of the same order of magnitudeas that required for the operation of a standard carbon microphone.FIGURE 23 shows an arrangement whereby the modulation of current fromthe direct current source by the carbon microphone 220 produces amodulated infra-red signal from the germanium source 222. It is thuspossible to construct very simple germanium infra-red sources forwireless communication at short distances.

FIGURE 24 shows a hand-held device for projecting and aiming a fine beamof infra-red radiation over short distances (5 meters to 200 meters) forpurposes of message transmission. An infra-red beacon 230 is composed ofan incandescent lamp 232 mounted in a housing behind infra-red filter234. Mounted on the top of this beacon is a germanium infra-red source236 connected to microphone 238 and amplifier 240. The entire assemblyis manipulated by handle 242 until the infrared radiation projected bythe beacon 230 impinges on the distant receiving station. The beacon isthen extinguished and the infra-red output of the germanium source 236is modulated by speaking into the microphone 238. The modulatedinfra-red signal is then received by a unit similar to that shown inFIGURE 25.

In the device represented by FIGURE 25 the mirror 244 receives theinfra-red radiation projected by the germanium source in the devicerepresented by FIGURE 24 and reflects it on to an infra-red sensitivephotoconductive cell 246 placed at the focal point of the mirror. Thesignal caused by the change in photoconductivity due to the impingingmodulated infra-red radiation is amplified by amplifier 248 and is fedinto speaker 250. The receiving device is mounted on a support 252. Aphotoradiator" 254 is also mounted on the support to assist theprojecting station in properly aiming the message transmitting device.It also may be advantageous to surround the receiving cell 246 withblack cellophane 256. This prevents visible light from impinging on thecell but allows infra-red radiation to pass. The apparatus representedby FIGURES 24 and 25 has many uses where a small device is required fortransmitting a signal from a station in motion, such as a truck, boat,locomotive, etc., to a receiving station.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States LettersPatent is:

1. A semiconductor infra-red source for producing a modulated beam ofinfra-red radiation comprising a body of semiconductor material havingonly one type of impurity atoms, a plurality of electrodes electricallyconnected to said body with a first group of said electrodes havingpoint contact with only one surface on said body in a small area thereoffor increasing the concentration of carries therein and at least oneelectrode having large area contact with said body, and means connectedto said electrodes for passing a modulated current through theelectrodes and said semiconductor body to cause increased emission ofinfra-red radiation from the region of said small area modulatedcorrespondingly to said current.

2. A semiconductor infra-red source for producing a modulated beam ofinfra-red radiation comprising a body of semiconductor material havingonly one type of impurity atoms, a plurality of electrodes electricallyconnected to said body with a group of said electrodes having only pointcontact within a very small area of said body and spaced closelytogether to concentrate carrier injection therein, and at least oneelectrode having large area contact with said body at a point spacedfrom said small area, and means connecting said electrodes to a commonsource of modulated current whereby the infra-red emission at all ofsaid point contact electrodes in said very small area is additive.

3. An infra-red source for producing a modulated beam of infra-redradiation comprising a body of semiconductor material having only onetype of impurity atoms, a plurality of point contact electrodes groupedclosely together to make contact with said body in a very small areathereof for concentrating the carrier injection in said small area andat least one electrode having a large area contact with said body at apoint spaced from said small area, means connecting a separate anddifierent source of 14 modulated electric current to different ones ofsaid point contact electrodes so that the infra-red emission in thesmall area is modulated correspondingly to the respective currentsource.

4. A semiconductor infra-red source capable of producing an undulatedbeam of infra-red radiant energy, said source comprising a body ofsemiconductor material transparent to infra-red radiation, a pluralityof electrodes electrically connected to said body with a group of saidelectrodes having point contact within a small area on said body andbeing spaced closely together and at least one of the other of saidelectrodes being a base electrode having large area contact with saidbody, said large area contact being spaced from and encircling saidsmall area, and means for passing an undulated electric current throughthe electrodes and said body to cause emission of infra-red radiation inthe region of said small area undulated correspondingly to said current,said body having a radiation exit surface formed on the side thereofopposite the surface containing said small area for directing theemitted radiation into a beam of predetermined geometric form.

5. A semiconductor infra-red source for producing and modulating aninfra-red radiant energy beam comprising a conically shapedsemiconductor crystal having formed on the end thereof opposite theapical end an optically curved radiation exit surface, a base electrodeplate having an aperture therein fitted to said crystal with theaperture walls in contact with the conical crystal intermediate theapical and opposite ends thereof, and a point contact electricallycontacting the apical end of said conical crys tal and mounted to saidbase electrode plate and electrically insulated therefrom.

6. The infra-red source defined in claim 5 wherein said point contactelectrode is resiliently mounted to said base electrode plate.

7. The infra-red source defined in claim 5 wherein said base electrodeplate and point contact are mounted to said crystal by an infra-redtransparent member having a recess therein receiving at least a portionof said crystal optically curved surface and including meansmechanically connected to said base electrode plate to maintain the samein place on the crystal.

8. The infra-red source defined in claim 7 wherein said infra-redtransparent member is so shaped and has an index of refraction such thatsaid crystal and member together constitute a substantially achromaticdoublet.

Briggs Oct. 26, 1954 Aigrain Nov. 18, 1958

1. A SEMICONDUCTOR INFRA-RED SOURCE FOR PRODUCING A MODULATED BEAM OFINFRA-RED RADIATION COMPRISING A BODY OF SEMICONDUCTOR MATERIAL HAVINGONLY ONE TYPE OF IMPURITY ATOMS, A PLURALITY OF ELECTRODES ELECTRICALLYCONNECTED TO SAID BODY WITH A FIRST GROUP OF SAID ELECTRODES HAVINGPOINT CONTACT WITH ONLY ONE SURFACE ON SAID BODY IN A SMALL AREA THEREOFFOR INCREASING THE CONCENTRATION OF CARRIES THEREIN AND AT LEAST ONEELECTRODE HAVING LARGE AREA CONTACT WITH SAID BODY, AND MEANS CONNECTEDTO SAID ELECTRODES FOR PASSING A MODULATED CURRENT THROUGH THEELECTRODES AND SAID SEMICONDUCTOR BODY TO CAUSE INCREASED EMISSION OFINFRA-RED RADIATION FROM THE REGION OF SAID SMALL AREA MODULATEDCORRESPONDINGLY TO SAID CURRENT.